1. Field of the Invention
The invention relates generally to fibre optic communications and more particularly relates to a supervisory network for providing differentiated services for an optical transport system.
2. Related Art
Driven by the dramatic growth rate of the Internet, the demand for the optical transport systems is increasing exponentially in bit rates and transmission distance. The expression “optical transport system”, as used herein, relates to any system which uses one or more wavelengths to communicate information across an optical fiber, and uses any number of amplifiers along the fiber to boost the signals. Such optical systems include, but are not limited to, metro, long haul, and ultra long haul optical transmission systems, cable television systems, and local area networks (LANs). Optical transport systems range from low capacity 32 wavelengths, 622 Mb/s per wavelength short reach point-to-point systems, to more complex 100 wavelengths, 10 Gb/s per wavelength, 4000 Km reach systems. Several laboratory and field experiments have also demonstrated long haul optical transmission at speeds of 40 Gb/s and higher.
The nodes of an optical transport network comprise multiplexers which consolidate a plurality of information-carrying channels into a multichannel signal (or a dense wavelength division multiplexed D/WDM signal), and demultiplexers that separate the multichannel signal into its components. The transport network may also comprise electrical or optical cross-connect nodes, transmitter and receiver terminals, optical amplifier nodes, electrical regenerator nodes, and other equipment specific to the physical optical layer.
ATM (asynchronous transfer mode) is a dedicated connection switching technology, which establishes a specific path, called a switched virtual circuit (SVC), between a source and a destination node. Every ATM cell flowing from a source switch to a destination switch travels over the same SVC. Such an arrangement allows the network to establish a specific quality of service (QoS) for a specific connection, by reserving resources in advance when the SVC is created. The ATM is generally carried over WDM or SONET as the physical layer.
Packet oriented networks such as Internet, transmit packets from a source to a destination via network routers. IP is called a “connection-less” technology, because each packet of information can take a different path to reach the destination node. At the source router, the information to be transmitted (text, video, audio, etc., is divided into a number of packets, which are placed in queues and then forwarded to the destination routers. The packets travel through a number of nodes/routers and when they arrive at destination, they are assembled to produce the information that was originally transmitted. However, since they travel along diverse routes, they arrive at the destination node with different delays, and have to be re-ordered. The actual transmission delay and delay variations affect both resource capacity and resource utilization at each network element of a communication path.
A connection-less system may be unreliable due to the packet loss, reordering and duplication, which often exhaust a router and results in packets being discarded (packet loss). The IP delivery model is often referred to as a “best effort” system and an additional end-to-end protocol such as transmission control protocol (TCP) is required to provide reliability. TCP achieves this through mechanisms such as packet retransmission, which adds to the overall information transfer delay.
IP can be carried over ATM. Due to the differences between the IP and ATM, various protocols were created to transmit IP traffic over ATM networks, such as NHRP (next hop resolution protocol) and RRSP (Resource reSerVation setup protocol). IP can also be carried directly by the WDM physical layer.
Quality of Service (QoS)
One method of prioritizing traffic in a telecommunications network is to use QoS performance requirements, which refer to delay tolerances, delay variance, and data loss requirements. Applications like automatic laser safety shutdown, inter-node distributed control loops, etc., have extremely low latency and loss requirements, and therefore these applications must take high priority. Applications like remote software download, remote Telnet sessions, etc., can tolerate high latency and data loss, and therefore these applications take low priority.
It is known to insert QoS information in the header of a data unit allowing the device receiving and re-transmitting such data unit to examine the QoS portion of the header and to assign priority accordingly. The QoS allocation is typically applied in a static manner. Still, protocols like RSVP and MPLS (multi-protocol label switching) have capabilities to assess network capacities dynamically and reserve equipment resources to fulfill a specific application service requirements.
Generally, each router supports “n” priority classes of service (CoS), n>1, with “n” allocated to the lowest priority class known as “best effort”, and “1” to the highest priority class known as “guaranteed delivery”. The best effort model is adequate for some applications such as file transfers and e-mail. For other applications however, such as using multimedia information, the delay provided by the best effort model is unacceptable. For these applications, a method of ensuring a certain quality of service QoS, including guaranteed bandwidth, delay and packet loss is required.
A QoS manager typically controls assignment of incoming traffic to priority classes, monitors forwarding delays, and restricts admission of new traffic to compensate for states of congestion. Other QoS managers maintain separate queues for a given priority. A lower priority queue is served only after all packets in the higher priority queue have been transmitted/served.
All types of data can be characterized in terms of tolerance to data loss and delays during transmission. Latency is the delay suffered by a packed during its travel. It can be measured end-to-end (network latency), from the time when a data unit is produced at a source to the time it reaches the destination. It can also be measured from the time the packet arrives at a router until it leaves for the next hop (router/switch latency). The data flow rate is the number of data units per second that are processed by a processing node.
Without sufficient resources and proper resource management, data flows may lose their data or timelines in a random fashion, causing undesired levels of distortion for packets arriving at destination. Loss of data is the main concern and is due to excessive delays which exhaust the resources of a switch/router resulting in packets being discarded.
Supervisory Channel
The term “supervisory channel”, or optical supervisory channel (OSC) as used herein, relates to a channel using a dedicated optical wavelength which runs between two nodes and is used for monitoring and control purposes within a optical transport system. There can be more than one such channels between two nodes. Bidirectional supervisory channels are also used for inter-node communication within the optical transport system, to provide access to all nodes in the system.
An example of such a supervisory channel dedicated to monitoring the system performance is disclosed in U.S. Pat. No. 5,798,855 issued on Aug. 25, 1998, to Alexander et. al. The monitoring channel of Alexander et al. operates at a wavelength outside the band of an optical amplifier in order to survive the failure of the optical amplifier.
The current transport networks have to support multiple types of applications, some which are mission critical. The current transport networks do not provide differentiated QoS for different network applications. Such mission critical applications either were not existent before, or were handled through dedicating large bandwidth, resulting in a waste of resources and increase in cost.
There is a need for a mechanism to ensure that communication among applications of an optical transport network meets the QoS objectives for efficiently utilizing the network resources.